In the prior art temperature controlled radiating fan, a pulse width modulation (PWM) serves to change the modulation way of the rotary speed of the fan motor, such as those disclosed in Taiwan Patent No. 0327486, "DC brushless fan PWM control circuit", No. 0152583, "PWM DC brushless radiating fan", and No. 0146147, "AC motor stageless speed change PWM control".
However, the prior art designs have the disadvantages of larger volume, many components, high manufacturing cost, worse linearity. Thus, there is an eager demand for a temperature controlled radiating fan having a control circuit which is varied linearly according to a voltage variation.